Effects of control structure on performance for an automotive powertrain with a continuously variable transmission

The wheel speed control problem of an automotive powertrain equipped with a conventional spark-ignition engine directly connected to a continuously variable transmission (CVT) and an electronic throttle is considered. We revisit the work by Guzzella and Schmid (1995) and show that the control structure that dedicates the throttle actuator to maintaining engine operation at the maximum fuel efficient operating points results in a single-input-two-output system that presents a fundamental limitation in the achievable wheel speed response. The limitation arises from the nonminimum phase (NMP) zero in the transfer function from the CVT ratio rate to the vehicle wheel speed. We relax the requirements on the fuel efficient operation and employ the electronic throttle as a second actuator for the wheel speed regulation problem. The resulting two-input-two-output control structure is then analyzed to determine how to mitigate the limitations associated with the NMP zero. Simulations show that the multivariable strategy improves the system performance because it produces minimum phase behavior without large transient deviations from the optimal fuel economy operation.